Conventionally, a tire comprises a tread intended to be in contact with a road while a vehicle provided with tires is being driven. This tread is provided with a tread pattern formed by raised elements, these elements delimiting grooves for draining water present on the road during wet weather. However, the presence of grooves, and notably of circumferentially oriented grooves, is a source of rolling noise.
A block is a raised element formed on the tread, said element being delimited by voids or grooves and comprising lateral walls and a contact face intended to come into contact with the roadway. This contact face has a geometric center defined as being the barycenter or center of gravity of the face.
A rib is a raised element formed on a tread, this element extending in the circumferential direction and making a complete circuit of the tire. A rib comprises two lateral walls and a contact face, the latter being intended to come into contact with the roadway during rolling.
A radial direction is understood to mean in the present document any direction which is perpendicular to the axis of rotation of the tire (this direction corresponds to the direction of the thickness of the tread).
A transverse or axial direction is understood to mean a direction parallel to the axis of rotation of the tire.
A circumferential direction is understood to mean a direction tangential to any circle centered on the axis of rotation. This direction is perpendicular both to the axial direction and to a radial direction.
A groove denotes the space delimited by facing walls of material, these walls being connected together by a groove bottom. Under the usual rolling conditions, the walls delimiting this groove cannot come into contact with one another.
The tread surface of a tread corresponds to the surface of the tread that is intended to come into contact with ground during the rolling of a tire provided with such a tread.
It is known that, as a tread enters the contact region in which it is in contact with a road during rolling, air is made to circulate in each groove. With the road, each groove forms a sort of tube having two open ends, the length of the tube corresponding to the length of the contact patch.
The air contained in this tube forms a column of air that is made to vibrate during rolling and the resonant frequency of which depends on the length separating the two ends of the tube and consequently on the length of groove in contact with the roadway.
This resonance of the air in the grooves has the consequence of generating, in a vehicle equipped with these tires, a noise inside the vehicle and a noise outside the vehicle. These inside and outside noises frequently correspond to a frequency close or equal to 1 kHz, which corresponds to a frequency to which the human ear is particularly sensitive.
In order to reduce such resonance noise, it is known (see for example the patent document FR2715891) to arrange in each groove of circumferential or generally circumferential orientation, a plurality of noise attenuating devices, each device being composed of membranes or relatively thin flexible blades of rubber compound, each membrane or flexible blade occupying the entire cross section of the groove or at least a large portion of this cross section so as to form a closing device. The installation of these flexible blades changes the length of each air pipe and thus makes it possible to change the resonant frequency. This results in a change in the perceived noise. The elastic nature of the material of each flexible blade also ensures sufficient return forces for returning the flexible blades into a position closing the groove when liquid is no longer flowing in said groove.
Each flexible blade can extend from the bottom of the groove or be fixed to at least one of the walls delimiting said groove. Relatively thin is understood to mean that each flexible blade is able to flex in order to at least partially open the cross section of the groove under the effect of a flow of liquid notably when driving in the wet. These same blades remain in the position closing the groove when driving in the dry. As has been recalled above, the devices formed of flexible blades that are described in the prior art—on account of the need to be able to flex under a flow of liquid—are connected either to the bottom of a groove or to one of the lateral walls delimiting a groove.
By virtue of these flexible blades, the length of the column of air in each circumferential groove is reduced compared with the overall length of the groove in the contact, and this results in a change in the resonant frequency. The shift in frequency is toward resonant frequency values to which the human ear is less sensitive.
In devices formed of a single blade carried by the bottom of a groove of given depth, it is sometimes difficult to reconcile good molding and good flexibility of the blade. This is because the greater the height of the flexible blade and the finer the inflow of material during molding, the smaller the thickness of each flexible blade. This state of affairs causes a person skilled in the art to make certain compromises, notably with regard to the thickness of the flexible blade. This results in greater rigidity in the new state for each flexible blade, this increase in rigidity being even more appreciable in the part-worn state. The same type of drawback is encountered with devices formed of flexible blades carried by one or two lateral walls.
The documents JP 2007/210569 A and DE 44 03 662 A1 describe devices comprising several flexible blades.
The document EP 908330-B1 discloses a noise reducing device, said device being formed by three flexible blades, two of which are fixed to the walls delimiting a groove, and the third of which is fixed to the bottom of this groove. In this arrangement, the blade fixed to the bottom extends as far as the tread surface in the new state and raises the same production problems as those already mentioned. Furthermore, the two other blades are not sufficiently flexible.
The expression “lateral wall delimiting a groove” is interpreted in the present document as denoting one or the other of the facing lateral walls, these walls being connected together by a groove bottom. The height of the groove bottom corresponds to the legal wear limit indicated by at least one wear indicator.